Generally, it is known that two intake ports are provided to an engine cylinder, where a tangential port is used as one of the intake ports, while a helical port is used as the other intake port.
JP2009-019613A discloses that a swirl flow can be generated in an engine cylinder by two intake ports being a tangential port and a helical port, and a cylindrical member is provided upstream of the tangential port, to give a rotational component to intake air which flows out of the port. By giving the rotational component to the intake air flowing out of the tangential port, turbulence is produced in the intake air flow near an inner circumferential surface of the cylinder to reduce heat dissipation from the cylinder wall surface.
JP2000-64843A discloses that an opening edge of an intake port which opens in a cylinder is chamfered into a conical surface, i.e., the opening of the intake port is gradually increased in the aperture diameter to be a tapered opening (FIG. 1).
When the swirl flow is generated inside the engine cylinder, it is advantageous in stimulating the mixing of fuel and intake air. Not only the shape of the intake port but the length of the intake port plays an important role for generating the swirl flow. That is, since an acceleration distance of the intake air flowing into the cylinder becomes longer as the length of the intake port increases, an increased flow velocity of the intake air and a stronger swirl flow can be generated with the extended length of the intake port.
However, the extension of the intake port may sometimes be difficult depending on the engine design. One such design is to extend the intake port to each cylinder by branching one common port which opens to a side surface of the cylinder head into two intake ports. In this case, the acceleration distance of the intake air becomes substantially shorter compared to intake ports that are completely independent from the side surface of the cylinder head to the cylinders. Since the flow passage cross-sectional area of the common port is large, it does not have a large effect of accelerating the intake air. Therefore, it is difficult to generate an expected, strong swirl flow in the cylinder.
For example, such a common port is provided in the following case. In this case, a common rail is provided to the cylinder head, a fuel pump is provided on the cylinder block side, and fuel piping which connects the fuel pump with the common rail is provided to the side surface of the cylinder head where the intake port opens. Here, a fuel-feed pipe and a fuel-return pipe need to pass through a space between openings of the adjacent intake ports. In addition, if the independent intake ports are adopted, it may be difficult to secure the space for the fuel piping because of the intake port openings. In order to solve this problem, two intake ports connected to the respective cylinders are joined to have one inlet to secure the piping space, which may be referred to as a “common port system.” However, it is difficult in the common port system to generate the strong swirl flow because of its substantially short acceleration distance of the intake ports as described above.